The invention relates to an air spring system for a motor vehicle including a compressor supplying compressed air by way of an air dryer to a compressed air storage chamber and to individual air springs by way of vehicle level control valves.
An air spring system of this type is known from DE 198 35 491 A1. When the vehicle body is being lowered, switching valves assigned directly to the vehicle wheels and a further switching valve are electromagnetically actuated by means of a control unit, with the result that a line which originally is connected to the atmosphere is connected to the pressure line. The increase in pressure obtained thereby in this line opens a switching valve which is arranged parallel to a non-return valve, so that air can be discharged from the air springs.
It is the object of the present invention to provide an air spring system for motor vehicles in which air can be quickly discharged from the springs when necessary.
In an air spring system of a motor vehicle having a pressure line extending between a compressor and a pressure storage or air springs of the motor vehicle, the pressure line includes a section between a dryer and the air springs with a variable flow cross-section and with one electromagnetically actuable switching valve arranged in parallel with the variable flow cross-section and providing unrestricted flow when opened.
The basic idea of the invention is to provide a section of the pressure line between a dryer connected downstream of the compressor and the level control valves of the air springs with a variable flow cross section for the gas flow. The rate at which the air is discharged via the level control valves when the vehicle level is lowered can be adjusted by means of this variable flow cross section, which acts as a throttle. The maximum flow cross section is in his case selected to be smaller than the flow cross section of the switching valve which blocks the vent line. As a result, the compressed air is throttled in the pressure line section before flowing through the dryer. The throttling causes the relative humidity of the air to decrease, with the result that the expanded air, as it flows through the dryer, is able to take up the atmospheric moisture which has collected there whereby the dryer is regenerated. The dryer dehumidifies the compressed air, for example by means of a gel which collects the moisture from the air.
The pressure line section has a non-return valve which is arranged in parallel with the throttle of variable cross section. This non-return valve prevents air flow in the direction toward the dryer but opens for supplying air compressed by the compressor to the air springs or to a pressure reservoir. As a result, the throttle can be bypassed and in this way the power needed for operating the compressor can be lower so that also the compressor may be smaller. The variable flow cross section of the throttle may also be increased to its maximum during compressor operation.
A further possible way of widening the flow cross section in the pressure line section is provided by a configuration of the invention, in which a switching valve, for example an electromagnetic switching valve, is arranged in parallel with the throttle, the switching valve closing or opening a bypass passage to the throttle. As a result, when it becomes necessary to lower the vehicle body quickly, the throttle can be effectively bypassed.
In a particular embodiment of the invention the switching valve includes a throttle such that, in its closed position the switching valve can act as a throttle in the pressure line and in its operating position can provide for a virtually non-throttled pressure line.
By means of joint electronic actuation of both the electromagnetic switching valve of the pressure line section and that of the vent line, it is possible to implement a simple and inexpensive configuration of the invention. By way of example, both switching valves can be actuated by means of the same output stage of a control unit, with the result that an additional output stage is not necessary. For this purpose, the switching valve of the pressure line section is switched already by a low excitation current or a low voltage, whereas the switching valve of the vent line is switched only at a higher excitation current or a higher voltage. For normal lowering of the vehicle body at a relatively low lowering rate, the control unit actuates only the switching valve of the vent line whereby the air is discharged via the throttles in the pressure line section, the dryer and the vent line. If it is necessary to lower the vehicle body more quickly, both switching valves are actuated by a higher excitation voltage or a higher excitation current, so that in addition to the switching valve of the pressure line section the switching valve of the, vent line is also opened. The air can then bypass the throttled pressure line section via the vent line.
In an embodiment which is very favorable in terms of energy use the switching valve in the vent line is a pressure-controlled switching valve. In this case, the actuator of the valve is on one side subjected to the pressure prevailing in the vent line, so that the valve opens automatically at a defined over-pressure. On the other side, the pressure of a pilot line, which can be connected to the pressure line via an electromagnetic pilot valve, acts on the actuator of the valve. As a result of the pilot valve being actuated and switched by means of the only output stage of the control unit, which also controls the valve in the pressure line section, the pressure prevailing in the pressure line opens the switching valve of the vent line. Consequently, a lower electrical power is required to open the switching valves in the vent line.
In a further embodiment the vent line is in communication with the intake line. This embodiment represents a variant which is very advantageous for acoustic reasons, in particular if the vent line is connected to the intake line between intake filter and compressor in the intake line.
Various embodiments of the invention will be described below with reference to the accompanying drawings. Throughout all the figures, identical components are provided with identical reference symbols.